I am trying to build an EMF detector or non-contact voltage detector based on an Arduino to be used as a prop. The detected EMF value should be read from an analog input and based on the level, 1-5 LEDs should light up. I found two circuit designs which didn't look too complicated.
The first "circuit" basically consists of a floating pin / wire which is pulled to ground with a high value resistor (1 or 3.3 MΩ).
However this design didn't work for me, I was able to detect turned on lamps, extension cords somehow. But the circuit wasn't very reliable. Often it triggered once and stopped after that or turned on randomly. I tried various resistor values and "antenna" types like copper plates, long wires and coils.
I had much better results with the following circuit. First I didn't use the Arduino at all and it worked perfectly, although the range was very limited. I also didn't use the 1 MΩ and 100 kΩ resistors and I don't really understand the purpose of them. Limit the current to avoid accidental triggering?
Now, instead of turning on a LED with the second circuit, I want to somehow measure the current at the third transistor. My first idea was to remove the LED and measure the voltage at the 220 Ω resistor in reference to ground. This didn't work at all, I always measured 5 V, which makes sense I guess. Do I need two resistors instead to create a voltage divider?